Normally, a touch panel with a transparent surface is mounted on a display device, such as the liquid crystal display of a notebook computer or a personal digital assistant (PDA), which serves as an input device or an input interface by a user without an additional keyboard or mouse. The touch panel has been used for a graphic process such as CAD. The touch panel is referred to as a touch film, a touch screen, a digitizer, a tablet or an electric graphic input panel (EGIP).
Depending upon the sensing method of a touch panel when a user touches its surface, touch panels are usually classified as either a resistive type, a capacitive type, or an electromagnetic type. In the resistive type touch panel, a current change is detected according to the position of a touching point by applying a DC voltage. Meanwhile, in the capacitive type touch panel, the position of a touching point is detected by a capacitance coupling with applying an AC voltage. Also, in the electromagnetic type touch panel, the position of a touching point is detected by detecting a resonant frequency resonated as an induction voltage by applying an electromagnetic field.
The respective type touch panels have different signal-amplifications, resolutions, designs and processing technology characteristics, so that the touch panel type is selected according to the desired use of the display device using the touch panel and by considering the economical efficiency, endurance and electro-optics, electrical, mechanical, environment-resisting, and input characteristics.
However, touch panels with a transparent surface mounted between the user and the viewing surface of a display, such as a liquid crystal display, have several drawbacks. For example, the transparent surface, together with other layers between the liquid crystal material may result in multiple reflections which decreases the contrast of the display and produces glare. Moreover, externally adding a touch panel to the display increases the manufacturing expense of the display and increases the complexity of the display. Therefore, some of the liquid crystal displays use and combine photo-sensitive elements on the TFT array substrate of the liquid crystal display instead of employing a touch panel mounted on the top surface of the liquid crystal display so that the assembly process of the liquid crystal display with a touch function is simplified.
Referring to FIG. 1A, a current type photo-sensitive element 100 includes a photo thin film transistor 110 (photo TFT) and a switch thin film transistor 130 (switch TFT). The switch TFT 130 electrically connects to the readout line 140 with the source electrode 136 thereof and the switch line 150 with the gate electrode 132 thereof. The drain electrode 134 thereof electrically connects to the source electrode 116 of the photo TFT 110. In addition, the gate electrode 112 and the drain electrode 114 both electrically connect to a bias voltage line 120. The bias voltage line 120 provides a voltage to the photo TFT 110 so that the photo current of the photo TFT 110 is adjusted by the brightness sensed by the photo TFT 110 while the switch TFT is turned on. Normally, the photo current is proportional to the brightness sensed by the photo TFT 110. However, there are so many metal lines, such as the readout line 140, the switch line 150 and the bias line 120, are formed in the TFT array substrate so that the aperture ratio of the liquid crystal display is reduced.
The current type photo-sensitive element 100 has to connect to at least three metal lines, such as the switch line, bias line and readout line, for driving the photo-sensitive element 100 to measure the brightness thereon. Thus, the current type photo-sensitive element is also referred to a three-terminal type element. Referring to FIG. 1B, a charge type photo-sensitive element 800 is described. The photo-sensitive element 800 includes a photo thin film transistor 850 (photo TFT), a switch thin film transistor 840 (switch TFT), and a capacitor 860. The switch TFT 840 electrically connects to the readout line 810 with the source electrode thereof and the switch line 820 with the gate electrode thereof. The drain electrode thereof electrically connects to the source electrode of the photo TFT 850. In addition, the gate electrode and the drain electrode of the photo TFT 850 both electrically connect to a bias voltage line 830. It is noted that the discharge type photo sensitive element is also referred to a three-terminal type element and differing from the current type photo sensitive element 100, the charge type photo-sensitive element 800 has a additional element, the capacitor 860.
With the same manner, the charge type photo-sensitive element 800 also has to connect to at least three metal lines, such as the switch line, bias line and readout line, for driving the photo-sensitive element 800 to measure the brightness thereon.
Therefore, the liquid crystal display combining photo-sensitive elements on the TFT array substrate results in more metal lines in the liquid crystal display reducing the aperture ratio thereof. Accordingly, there is a need to provide a high sensitive touch panel in the liquid crystal display but not to significantly reduce aperture ratio thereof.